Known methods for preparing 1,1,1,2,3,3,3-heptafluoropropane include, for example, a method comprising reacting hexafluoropropene with hydrofluoric acid in the presence of an antimony catalyst (Japanese Unexamined Patent Publication No. 24362/1996), and a method comprising reacting hexafluoropropene with hydrofluoric acid using a hydrogen fluoride adduct of an organic nitrogen base as a reaction medium (Japanese Unexamined Patent Publication No. 53543/1998).
Hexafluoropropene used as a starting material in these reactions is highly useful as a fluororesin monomer and as an intermediate for fluorochemicals, and is expensive. Therefore, the above methods have a cost disadvantage.
Perfluoroisobutene (hereinafter referred to as "PFIB") is a byproduct in the manufacture of hexafluoropropene. PFIB is a highly toxic compound which is usually converted to the less harmful methanol adduct. Effective uses of the PFIB-methanol adduct, however, are not generally known.
Japanese Unexamined Publication No. 309791/1995 describes a method for preparing hexafluoropropane, perfluoropropane and heptafluoropropane using PFIB as a starting material. The method comprises the steps of reacting PFIB-alkanol adducts (e.g., 2H-octafluoroisobutyl methyl ether and heptafluoroisobutenyl methyl ether) with trialkylamine and water to produce hexafluoropropane and electrolyzing and fluorinating hexafluoropropane to give perfluoropropane and heptafluoropropane. According to this method, only a small amount (about 10%) of heptafluoropropane is obtained as a byproduct.
WO 98/28247 describes a method for preparing heptafluoropropane, which comprises reacting hexafluoropropane with fluorine gas. This method also produces a large amount of byproducts, such as perfluoropropane.
U.S. Pat. No. 3,399,179 and J. Org. Chem., 34, 2446 (1969) describe a method of reacting carboxylic acid with fluorine gas to replace carboxyl groups with fluorine atoms. This method comprises bringing carboxylic acid or a metal salt thereof into contact with fluorine gas in the presence of a diluent such as water or alcohol to produce fluoride. However, it has been revealed that 2-trifluoromethyl-3,3,3-trifluoropropionate can not be fluorinated by this method.